This invention relates to a fuel tank device for a vehicle which is mounted on an upper portion of a fuel tank, and includes a float valve for discharging fuel vapor from the interior of the fuel tank to the exterior thereof during refueling and for limiting the flow of fuel out of the fuel tank when the fuel tank is filled with the fuel, and a diaphragm valve for feeding the outflow fuel vapor into a canister.
Conventional float valves of the type described are disclosed, for example, in Japanese Patent Unexamined Publication No. 5-185850 and Japanese Utility Model Examined Publication No. 4-39061, and such a float valve has a construction as shown in FIG. 10.
This float valve 100 comprises a cylindrical casing 102 with a closed bottom, which is formed by a top wall 102a, a side or peripheral wall 102b and a bottom wall 102c, a float 110 received in a float chamber 104 formed in the casing 102, a valve portion 110a formed on an upper surface of the float 110 for closing an air outflow passage 111, and a compression coil spring 114 provided between a lower surface of the float 110 and the bottom wall 102c. Through holes 118 and 118b for flowing the air and fuel vapor therethrough into the float chamber 104 are formed through the side wall 102a and the bottom wall 102c of the casing 102, respectively.
In the float valve 100 of this construction, during the supply of fuel into a fuel tank FT, the air and fuel vapor flows from the fuel tank FT to the exterior thereof by flowing through the through hole 118 in the casing 102, into the float chamber 104 and out through the air outflow passage 111. Then, when the fuel within the fuel tank FT reaches a full liquid level FL, the fuel flows into the float chamber 104 through the through hole 118b to impart buoyancy to the float 110, thereby floating the float 110. The float 110 thus rises, so that the valve portion 110a, formed on the upper surface of the float 110, closes the air outflow passage 111, thereby preventing the fuel from flowing out from the fuel tank FT.
In the above float valve 100, however, the full liquid level FL, which determines the timing of closing of the air outflow passage 111 by the valve portion 110a as a result of rising of the float 110, is predetermined, that is, set to a constant level, by a resilient force of the compression coil spring 114 of the float valve 100, the weight of the float 110 and the buoyancy produced by the float 110. Various kinds of fuel tanks FT have particular full liquid levels, and therefore the design of the float valve 100 must be changed in accordance with the full liquid level of each fuel tank. This is disadvantageous and costly.
A conventional diaphragm valve of the type described is disclosed, for example, in Japanese Patent Unexamined Publication No. 60-184779, and is used to protect a fuel tank against the increase of a vapor pressure within the fuel tank. This diaphragm valve will now be described with reference to FIG. 11. In FIG. 11, the diaphragm valve 200 includes a housing 202 whose internal space serves as a valve chamber 204. This valve chamber 204 is divided into a reference pressure chamber 206 and a fluid pressure chamber 208 by a diaphragm 210. An urging force of a compression coil spring 212 and the atmospheric pressure within the reference pressure chamber 206 are applied to one side or face of the diaphragm 210 while the pressure within the fluid pressure chamber 208 (which is created by the pressure of the fuel vapor fed into the fluid pressure chamber 208 through an inflow passage 214) is applied to the other side of the diaphragm 210. The forces acting respectively on the opposite sides of the diaphragm 210 are balanced with each other. Thus, when the diaphragm 210 moves out of engagement with a seat portion 216, the fuel vapor within the fuel tank flows to a canister (not shown) through the inflow passage 214, through an outflow passage 218 and through a discharge passage 222 in a discharge conduit 220.
In order to increase the flow rate in the diaphragm valve 200, it is necessary to increase the flow passage areas of the inflow passage 214, the outflow passage 218 and the discharge passage 222. If the flow passage area of the outflow passage 218 and the discharge passage 222 is narrowed midway therethrough, the vapor pressure, flowing through these passages 218 and 222, acts in a valve-opening direction, so that the opening and closing of the diaphragm 210 becomes unstable. Therefore, to avoid such unstable operation, the flow passage area of the discharge passage 222 needs to be large.
However, if the diameter of the discharge conduit 220 is increased in order to increase the flow passage area of the discharge passage 222, the height of the diaphragm valve 200 increases, which has resulted in a problem that the fuel tank device can not be easily mounted on a vehicle.